CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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n is the roughness coefficient of the canal (also called
Manning's n) which is dependent on the materials of the
canal. The value of n for different types of canal is given in
Table 4.1.
A is the cross sectional area up to the water surface level
inm2.
S is the slope of the energy grade line. The invert slope of the
canal is used for S since it is parallel to the energy grade
line at longer lengths. For example 1:500 (1 in 500) invert
slope is 1 m of drop in level in 500 m of horizontal canal
length.
Sometimes percentage (%) or fractions are also used to denote
the slopes. For example a slope of 1% means that there will be
a difference in level of 1 m every 100 m of horizontal distance.
The equivalents of the slope in fractions or decimals are given
by the following examples;
2% = 2/100 = 0.02 = 1 in 50
2 in 1000 = 2/1000 = 0.002 = 1 in 500
1.5% = 1.5/100 = 0.015 = 1 in 67
3.5 in 1000 = 3.5/1000 = 0.0035 = 1 in 286
R is the hydraulic radius. R = A/P
P is the wetted perimeter in m. This is the total length of the
bottom and the two sides of the canal up to the
water surface level.
4.3.3 SEDIMENT DEPOSITION IN CANALS
The velocity in each section of the headrace canal should be
high enough to transport any sediment entering that section.
Between the intake and the gravel trap a velocity of 1.5 - 2.0
m/s is recommended. Between the gravel trap and the settling
basin a lesser velocity is possible, but the sediment transport
capability should be checked using a simplified version of
Shield’s formula: d=11RS
where:
d is the size of particle transported in a canal, in m
R is the hydraulic radius, in m
S is the canal bed slope.
If the gravel trap is designed to settle particles larger than 2
mm, then the canal downstream of the gravel trap must be
able to transport particles up to 2 mm.
Research at Wageningen University in the Netherlands
demonstrated that the roughness is increased for channels
under 1 metre in depth, because of the turbulence created by
the side and bed surfaces. The research showed that the
following equations can be used to find the roughness
coefficient. H is the depth of water.
Well maintained channels with little vegetation:
n = 0.03/ H
H<1m
Channels with short vegetation:
n = 0.04/ H
H<1m
Heavily overgrown channels:
n = 0.08/ H
H<1m
In practice it is sensible to maintain short vegetation in order
to protect the banks of canals.
TABLE 4.2 Recommended side slopes for headrace canals
CANAL MATERIAL
SIDE SLOPE ( N = hfv )
Rock/conglomerate (hard to loose)
0 (vertical) to 0.5
Firm clay
0.25 to 0.5
Loam
1.0 to 1.5
Sandy clay, sandy loam
1.5 to 2.0
Silty sand, sandy earth
2.0 to 2.5
Loose sandy earth, porous earth
2.5 to 3
Gravely earth, stiff or loose conglomerate
0.5 to 1
Gravel and boulder mixed with earth (soft and loose)
1.5 to 2
Stone masonry in mud mortar
See Note 2
Stone masonry in cement mortar
See Note 2
Plain concrete
See Note 2
Note :
1 These values are for canals excavated in soil of low moisture content with water table below canal bed. Slopes need to be
flattened if these conditions are not achieved.
2 The sides of lined canals may be vertical (designed as retaining walls) or at the slope recommended for the underlying soil.